This invention relates to a statistical sampling apparatus and more particularly to a device for demonstrating random sampling procedures.
Statisticians have long used a Shewart bowl for simulated sampling for experimentation and training. The bowl is used to demonstrate such things as the reject rate for very large quantities of manufactured goods. That is, a certain number of a large quantity of manufactured goods will be above or below acceptable range of deviation from the ideal. Since it is not generally necessary or desirable to test every one of the products, statistical sampling is used to determine how large a sample is required for finding rejects.
The early Shewart bowl used colored beads in a bowl with a paddle having recesses. The paddle was used to scoop beads out of the bowl with a portion of the beads settling into the recesses. This simulated sampling from the large quantity of beads in the bowl to those that were selected on the paddle. The use of an open bowl with the paddle was subject to spillage, particularly when used for training use, and the loss of beads changed the known reject (defect) rate thereby necessitating recounting of the beads.
In order to prevent spillage, it has been proposed in U.S. Pat. No. 3,095,655 to Berglund, et al., issued Jul. 2, 1963 to use a closed box containing colored beads. The paddle of the Shewart bowl is replaced by two plates that have matching holes. One of the plates is shifted to an offset from the other by a spring. Pressing a lever on the outside of the box aligns the two plates and allows beads above the plates to enter the holes and pass through the shiftable plate into the fixed plate. The fixed plate has its top face directly against a transparent cover and the beads are held captive in the fixed plate when the shiftable plate is again moved out of alignment with the fixed plate. This approach uses the distribution of bead colors to set the expected reject rate, and in its commercial embodiment, uses patterns of drilled holes to facilitate a choice of sample size.
This approach does away with the problem of spillage attendant with the original Shewart bowl. However, the device has moving parts which can result in jamming of the unit by beads.
I have developed a much simpler approach which does away with the spillage problem while eliminating all moving parts.